1. Field of the Invention
The present invention is directed to an apparatus and method for winding wire coils onto a bobbin or spool. More particularly, the present invention is directed to an apparatus and method for winding two strands of wire on a bobbin in a configuration which is particularly advantageous for constructing linear variable differential transformers.
2. Brief Description of the Prior Art
Coil winding machines of various types are well known in the prior art. One basic type of coil winding machine of the prior art has a motor-driven, rotating chuck which holds a bobbin or spool onto which a strand of wire is to be wound. A wire guide mechanism is mounted in the proximity of the bobbin. The guide mechanism is moved in a reciprocating, back and forth motion along and parallel with the axis of rotation of the bobbin.
A variation of this type of coil winding machine has several wire guides, each of which is placed in a different position along the longitudinal axis of a bobbin or spool, so that several coils are wound simultaneously. After the windings are complete, the bobbin is usually cut to provide individual coils.
In another basic type of coil winding machine of the prior art, the bobbin or spool is held substantially motionless. One or more circularly moving or orbiting flyer arms deposit one or more strands of wire on the bobbin.
These and other examples of prior art coil winding machines can be found in U.S. Pat. Nos. 2,742,238; 3,801,029; 4,007,881; 4,256,268; and 4,553,705.
More specifically, among the foregoing patents, U.S. Pat. No. 2,742,238 describes a coil winding machine which winds a plurality of axially aligned coils on a single arbor.
U.S. Pat. No. 3,801,029 describes an apparatus and method wherein wires from two spools are wound onto a rotating mandrel to form primary and secondary windings on top of one another. An insulating layer separates the primary and secondary windings. The wires which form the primary and secondary winding procedures do not cross one another during the winding process.
U.S. Pat. Nos. 4,007,881 and 4,553,705 disclose examples of winding apparatus where the winding procedures are controlled by a computer.
U.S. Pat. No. 4,256,268 discloses a coil winding apparatus wherein two to four strands of wire may be simultaneously wound on a mandrel. In this apparatus a winding head uses separate rotating and reciprocating flyer arms for placing each strand of wire on the mandrel.
One important application of the coil winding devices of the prior art is the making of linear variable differential transformers, or "LVDTs", as they are known in the art.
As is well known, LVDTs are, basically, position sensors. They operate on the principle that a ferromagnetic core is moved through the interior of a hollow bobbin onto which a single primary and two secondary windings are placed. The magnitude of the magnetic coupling of the coils to one another depends on the position of the movable core. Voltage and phase shift measurements, which may be obtained by comparing the outputs of the two secondary windings with the input to the primary winding, are indicative of the position of the movable core.
In one relatively advanced form of prior art LVDTs, the pitch of each of the two secondary windings varies along the length of the bobbin. One secondary winding is, however, disposed on top of the other. A disadvantage of such an LVDT is that the two secondary windings are not equidistant from the core. Therefore, in order to obtain accurate measurements, the varying pitch of the secondary windings must be properly adjusted to compensate for the differences in the distances of the two secondary windings from the core.
In light of the foregoing, there is a definite need in the prior art for a coil winding apparatus and method which is capable of winding improved LVDTs. The present invention provides such apparatus and method.